Infrared detector systems operating in the medium wavelength infrared (MWIR) spectral band (3 to 5 .mu.m wavelength) typically require sophisticated tracking algorithms to accommodate the large and often dynamic changes in background information that result from the relatively high contrast and solar content of the radiation. Detectors operating in the preferred long wavelength infrared (LWIR) spectral band (8 to 12 .mu.m wavelength), however, can attain the same or greater thermal sensitivity with reduced signal processing complexity. As a result, infrared detection and tracking can be accomplished using smaller, more cost-effective sensors having LWIR focal plane arrays.
Unfortunately, LWIR focal plane arrays and multiplexing readout circuits have design constraints that can severely limit system performance. In the readout portion of a focal plane array, for example, the input amplifier cell circuitry that couples each detector to the corresponding readout site must perform several functions that are difficult to incorporate simultaneously in the small amount of cell "real estate" typically available on a signal processing chip. Ideally, a detector/amplifier cell of an FPA should include the following: 1) a detector interface stage that provides low impedance at a uniform operating bias; 2) a large charge handling integration capacitance; 3) a stage for uniform suppression of the background if integration capacity is inadequate; 4) low power pixel multiplexing and reset stages; and 5) an output stage adequate to drive the bus line capacitance for subsequent multiplexing at video rates.
Staring LWIR FPAs in formats up to 128.times.128 have been demonstrated in the prior art. These LWIR devices, however, are typically coupled to conventional MWIR readout circuits, which have several deficiencies that compromise system performance. For example, the limited charge handling capacity provides overall sensitivity no better than that achieved by a typical MWIR FPA. This negates one of the benefits of operation in the LWIR spectral band. Furthermore, prior art LWIR FPAs lack impedance buffering, which forces a reduction in detector cutoff wavelength (i.e., .lambda..sub.c no greater than about 9 .mu.m) and an increase in fixed pattern noise (i.e., spatial noise remaining after application of conventional two-point non-uniformity correction). Fixed pattern noise creates a visible mask in the imagery that obscures low contrast, high frequency information, thus degrading (i.e., raising) the minimum resolvable temperature (MRT) and compromising performance under adverse or discriminating conditions. Moreover, prior art devices lack capability for reducing pixel pitch and increasing pixel density. If the pixel pitch and detector/amplifier cell real estate are reduced in prior art devices, the performance limitations are further aggravated.
Given the current photolithographic state-of-the-art and the limited chip area available, there is insufficient detector/amplifier cell real estate for a readout circuit with conventional staring architecture to integrate all of the most important features such as low input impedance, uniform detector bias, and satisfactory charge storage capacity. However, because small cells are necessary for FPAs with high pixel counts, integrated readout circuits with reasonable die sizes, and compact optics, all the important functions of the readout circuit must be integrated in as little chip real estate as possible. Thus, there is a need for a multiplexer readout circuit with improved architecture having characteristics that are better optimized for use in a staring LWIR FPA.